The sixth arc, A Vision of Shadows, is coming in 2016, and while it seems reasonable to assume that Sparkpaw and Alderpaw will probably be the protagonists (alongside that ShadowClan POV character confirmed for book two), we have no idea who the villains could be.
Which characters would you like to see as villain in the sixth arc? What should their personalities be like? What should their endgames be?
Over on the official forum, there's a fairly prolific rumour that the villains will be Hawkfrost's son (named Diego, apparently) and Featherpelt of WindClan.
I'm quite attached to the idea of Featherpelt as the main villain myself. The theory for her is that she's one of the new WindClan kits Dovewing saw in The Forgotten Warrior, daughter to Swallowtail and Antpelt. During the Dark Forest battle, she was seriously traumatized when she watched WindClan warriors turned against their Clanmates to fight alongside the Dark Forest and saw the deaths of both her mother and aunt (Thistleheart). After this, Featherpelt grew to hate StarClan, the Dark Forest, and the Dark Forest trainees (especially Ivypool, who she blames for everything wrong in her life).
After some sort of cataclysm (possibly the lake cats completely losing contact with StarClan) throws the Clans into disarray, Featherpelt takes over as leader of WindClan and attempts to restore order by conquering the other Clans and uniting them all under her rule. Along the way, she takes her revenge by murdering all the Dark Forest trainees she encounters. (All of them except Ivypool. She has a lengthy plan for revenge planned out, which includes capturing Ivypool, torturing her, and killing Birchfall and Whitewing in front of her.) Featherpelt's eventual end goal is to destroy both StarClan and the Dark Forest completely.
I think that could be pretty cool, especially since WindClan is the only Clan that hasn't produced a Main Villain, and unless Star Flower does turn out to be the bad guy in Path of Stars, Mapleshade is our only female Main Villain so far. (I consider the Main Villains to be Tigerstar, Brokenstar, Scourge, Hawkfrost, Sol, Thistleclaw, Mapleshade, and whoever ends up as the final villain of Dawn of the Clans.)
What I don't want to see is the return of the Dark Forest as the main bad guys, since their story is finished, and they've had so much time in the limelight already.
I wouldn't mind Sol returning for the first couple of books, but he shouldn't be the main villain and he should die by the end of the arc.